Semi-natural habitat cover, edge density and landscape conservation context shape the occupancy, species richness and reproductive performance of cavity-nesting Hymenoptera in an agricultural landscape

Abstract Context Cavity-nesting Hymenoptera, including bees and wasps, provide essential ecosystem services such as pollination and biological control in agricultural landscapes. However, their populations are increasingly threatened by habitat loss, landscape simplification and agricultural intensification. As a result, conservation measures aimed at restoring semi-natural habitats and enhancing landscape heterogeneity have been widely implemented in agricultural regions, yet their effectiveness at the landscape scale remains insufficiently understood for cavity-nesting species. Objectives We examined how conservation context, semi-natural habitat cover and edge density affect occupancy, species richness, reproductive performance and parasitism of cavity-nesting Hymenoptera in an agriculturally dominated landscape. Methods In the Günz Valley (southern Germany), we deployed trap-nests at 16 extensively managed meadows using a paired design comparing sites embedded within landscapes dominated by conservation-managed habitats (core areas, CAs) with comparable nearby meadows in conventionally managed landscapes (non-core areas, non-CAs). We quantified the effects of conservation context, semi-natural habitat cover and edge density on Hymenoptera occupancy, bee and wasp species richness, reproductive performance of Osmia bicornis , and parasitism by Melittobia spp. Results Overall Hymenoptera occupancy did not differ between CAs and non-CAs but showed strong context dependency: occupancy increased with semi-natural habitat cover in non-CAs, whereas no relationship was detected within CAs. Wasp species richness was higher in CAs, while bee species richness did not differ between conservation contexts. Reproductive output of O. bicornis increased with semi-natural habitat cover across both contexts, and brood survival increased with edge density. Parasitism by Melittobia spp. increased with host availability and tended to be higher in non-CAs. Conclusions Semi-natural habitats and ecotones play key roles in supporting occupancy and reproductive performance of cavity-nesting Hymenoptera, particularly in intensively managed agricultural landscapes. Conservation clustering alone did not uniformly enhance bee responses but may buffer density-dependent parasitism. Effective conservation strategies should prioritise maintaining semi-natural habitats and structurally heterogeneous landscapes while considering trophic interactions that may influence reproductive success.